[CI][operator] increase long test timeout to 15m

from 10m. The test `TestRayClusterAutoscalerMaxReplicasUpdate/Create_a_RayCluster_with_autoscaling_enabled(Scale_down_maxReplicas_from_2_to_1)`
in `raycluster_autoscaler_test.go` sometimes fails at line 405.

```
Timed out after 600.001s.
Expected
    <int32>: 1
to equal
    <int32>: 2
```

Signed-off-by: David Xia <[email protected]>

Author: davidxia

.buildkite/test-e2e.yml

@@ -55,7 +55,7 @@
     - set -o pipefail
     - mkdir -p "$(pwd)/tmp" && export KUBERAY_TEST_OUTPUT_DIR=$(pwd)/tmp
     - echo "KUBERAY_TEST_OUTPUT_DIR=$$KUBERAY_TEST_OUTPUT_DIR"
-    - KUBERAY_TEST_TIMEOUT_SHORT=1m KUBERAY_TEST_TIMEOUT_MEDIUM=5m KUBERAY_TEST_TIMEOUT_LONG=10m go test -timeout 60m -v ./test/e2eautoscaler/raycluster_autoscaler_test.go ./test/e2eautoscaler/support.go 2>&1 | awk -f ../.buildkite/format.awk | tee $$KUBERAY_TEST_OUTPUT_DIR/gotest.log || (kubectl logs --tail -1 -l app.kubernetes.io/name=kuberay | tee $$KUBERAY_TEST_OUTPUT_DIR/kuberay-operator.log && cd $$KUBERAY_TEST_OUTPUT_DIR && find . -name "*.log" | tar -cf /artifact-mount/e2e-autoscaler-log.tar -T - && exit 1)
+    - KUBERAY_TEST_TIMEOUT_SHORT=1m KUBERAY_TEST_TIMEOUT_MEDIUM=5m KUBERAY_TEST_TIMEOUT_LONG=15m go test -timeout 60m -v ./test/e2eautoscaler/raycluster_autoscaler_test.go ./test/e2eautoscaler/support.go 2>&1 | awk -f ../.buildkite/format.awk | tee $$KUBERAY_TEST_OUTPUT_DIR/gotest.log || (kubectl logs --tail -1 -l app.kubernetes.io/name=kuberay | tee $$KUBERAY_TEST_OUTPUT_DIR/kuberay-operator.log && cd $$KUBERAY_TEST_OUTPUT_DIR && find . -name "*.log" | tar -cf /artifact-mount/e2e-autoscaler-log.tar -T - && exit 1)
     - echo "--- END:Autoscaler E2E Part 1 (nightly operator) tests finished"
 
 - label: 'Test Autoscaler E2E Part 2 (nightly operator)'